It is important to the function of an elevator that its load be known at all times - whether to determine if the car is overloaded so that the elevator brake is not released while under that load or to determine which floors should be serviced. Several types of elevator weighing systems have been used.
Elevator load weighing systems may be divided into two groups: those that place a sensor beneath the cab platform and those that do not.
All elevator cab support means are subject to three kinds of forces: the vertical force of gravity upon the load, a bending load in the support member, and a horizontal force caused by the elevator moving forward or backward or from side to side within the hoistway. The goal of a load weighing system is to measure only one of these forces--the gravity force upon the load. A load in the center of the cab will exert a bending moment on the cab supports. A horizontal force exerted upon the cab may also cause the cab supports to momentarily bend. Attempts at measuring the cab load must account for these bending forces.
Several systems have been designed with the loadweigher beneath the car. Some measure the deformation of resilient pads beneath the car platform, U.S. Pat. No. 4,078,623, others use loadcells, U.S. Pat. No. 4,330,836, and still others, U.S. Pat. No. 4,573,542 and U.S. Pat. No. 4,223,752, use strain gauges on the cab supporting members. None of them deal with the problem of the bending moments experienced by rod supports. A disadvantage of the deformation systems is the inaccuracy introduced by hysteresis in the elastomeric material used in the pads. Although systems that use strain gauges attached to the cab supports are desirable in that the cost of strain gauges is low; such systems, nevertheless, do not account for bending moments. Systems which use loadcells are undesirable simply because of the cost of a member which must both support the load and measure it. None of the above systems give the most precise load measurements since they do not account for the bending moment in the cab supports. One solution to this problem is to modify the above load weighing systems to subtract the bending moment from the output of the measuring transducer. This, however, requires additional components and circuitry.
Attempts at weighing the load have also been made from a point other than beneath the car. One system places a sensor on the elevator rope, United Kingdom Patent No. 924,276. This system has an advantage in that it does avoid the problem with the bending moment. It does not, however, account for the fact that stretching in the rope is different at different points in the rope and at different times. A second system, U.S. Pat. No. 2,761,038, places a microswitch on the crosshead and measures a threshold amount of bending in the crosshead. This system only determines if an overload exists and does not give a continuous measure of the load. A third system, U.S. Pat. No. 2,411,023, measures the deflection of the elevator car crosshead by positioning cantilever arms on the front and back of the crosshead; a sensor on the underside of the crosshead measures the weight on the beam: here, the elevator cab, frame, compensating ropes and traveling cable. Another system, U.S. Pat. No. 3,323,606, involves a rotating bedplate having hoistway ropes connected to the car and counterweight; as the bedplate rotates it rotates with the hoistway ropes connected to the car. The displacement of the rotating bedplate measures the load on the bedplate, namely the elevator car, hoist cables, traveling cables and compensating ropes. Another load weighing system, U.S. Pat. No. 3,610,342, operates by measuring the torque delivered to a brake disk; as the brake closes upon the disk a sensor indicates the torque on the disk, and therefore the elevator cab, hoist ropes, compensating ropes and travel cable.
What all of these systems give with one hand they take away with the other. These systems which do not measure load from beneath the cab do avoid the need to compensate for a bending moment; however, they require that compensation for another factor--the weight of the compensating ropes and traveling cable. The weight of compensating ropes and traveling cable will vary from one end of the elevator shaft to the other making the load measurement dependent upon where the car is within the shaft. Weighing the elevator at the top of the shaft requires weighing the entirety of both the compensating ropes and travel cable, while weighing the elevator at the bottom of the shaft will require weighing very little of them. A second disadvantage of these systems is the relative error encountered in attempting to distinguish the cab load from a measurement that yields the weight of the cab, car frame, hoist ropes, compensating ropes, and travel cable.
While all of the above systems are to some extent successful in weighing the elevator, none of them both avoids non-axial loads and is suitable for use in a pendulum car support assembly.